> I'm sorry, I try to better explain my point.> Let's suppose this is a ODEs I want to solve.> http://www.mathworks.it/help/matlab/ref/eqn1311792846.> png> > y' are derivatives w.r.t the time.> I could use a Runge-Kutta method to solve them from> t0 a the final time (please, see> http://www.mathworks.it/it/help/matlab/ref/ode45.html)> . Well done.> > Now let's suppose I would integrate my problem in> auxiliary variable, let' say E. It is only> theoretically.> (e.g. E = t^2 or E = sin(t),...). I am able to expres> t0 e in tf in terms of E.> How can I solve my ODE now? It is sufficient to> multiply y' with dt/dE and use my Runge Kutta?> > Thanks for your support